The main problem is that leakage current has started to become a problem[1]. Back in the day designers could just scale down features and rely on the reduced capacitance of the smaller areas to lower power usage enough to let them put in more logic. Unfortunately that only reduces active power, not leakage power. Transistors have started leaking more now that they're smaller. You can reduce leakage by lowering the voltage your processor operates at, but that also causes a reduction in the frequency that your transistors flip at because your logic voltage becomes smaller with respect to the transistor threshold voltage, meaning less current but unit of charge you have to move. Modern devices are also tending to run up against saturation velocity[2] now, limiting their switching speed still further.
You certainly could increase clock speed by increasing the voltage you put into a chip, and just accepting that you're going to have more leakage current and more wasted power. But we're already close to the edge of what chips can dissipate right now. You can try having less logic between clock latches, meaning you have a higher clock speed for your switching delay. However, this increases the ratio of latches to everything else so its a matter of diminishing returns. It also means that you've pushed your useful logic further apart, and now you have more line capacitance too. Finally, you can decrease the temperature of your silicon to substantially reduce the amount of leakage you get. This will let you raise the voltage safely, letting you attain faster switching speeds. The only problem is that this requires expensive cooling devices.
You certainly could increase clock speed by increasing the voltage you put into a chip, and just accepting that you're going to have more leakage current and more wasted power. But we're already close to the edge of what chips can dissipate right now. You can try having less logic between clock latches, meaning you have a higher clock speed for your switching delay. However, this increases the ratio of latches to everything else so its a matter of diminishing returns. It also means that you've pushed your useful logic further apart, and now you have more line capacitance too. Finally, you can decrease the temperature of your silicon to substantially reduce the amount of leakage you get. This will let you raise the voltage safely, letting you attain faster switching speeds. The only problem is that this requires expensive cooling devices.
[1]http://en.wikipedia.org/wiki/Leakage_%28semiconductors%29 [2]http://en.wikipedia.org/wiki/Saturation_velocity